Name : Nashir Idzharul Huda NIM : 21100113130090

Geological Engineering B

Surfaces and Interfaces: General Concepts
For purposes of terminology, it is common practice to refer to that nebulous region as a ‘‘surface’’ or an ‘‘interface.’’, In general, however, one usually finds that the term ‘‘surface’’ is applied to the region between a condensed phase (liquid or solid) and a gas phase or vacuum, while ‘‘interface’’ is normally applied to systems involving two condensed phases. There are several types of interfaces that are of great practical importance and that will be discussed in turn. These general classifications include, solid– vacuum, liquid–vacuum, solid–gas, liquid–gas, solid–liquid, liquid– liquid, and solid–solid. A list of commonly encountered examples of these interfaces is given in a table below Interface Type Solid–vapor Solid–liquid Liquid–vapor Liquid–liquid Occurrence or Application Adsorption, catalysis, contamination, gas–liquid chromatography Cleaning and detergency, adhesion, lubrication, colloids Coating, wetting, foams Emulsions, detergency, tertiary oil recovery
TABLE 2.1. Common Interfaces of Vital Natural and Technological Importance

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as in a petroleum–seawater emulsion. nature will always act so as to attain a situation of minimum total free energy. In order for such a boundary to be stable it must possess an interfacial free energy such that work must be done to extend or enlarge the boundary or interface. Such an effect may be beneficial. as in the case of a cosmetic emulsion. In the case of a twophase system. there must be a region through which the intensive properties of the system change from those of one phase to those of the other. the interfacial energy will still be positive. as for example in the boundary between a solid and a liquid.In order for two phases to exist in contact. the rate at which area changes occur. to some extent. and other factors. Although thermodynamics is almost always working to reduce interfacial area. In order to define an interface and show in chemical and physical term that it exists.
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. but the changes caused by the alteration may prolong the ‘‘life’’ of any ‘‘excess’’ interfacial area. we have access to tools that allow us to control. In this way one can obtain an idea of the situations and events occurring at interfaces and have at hand a set of basic mathematical tools for understanding the processes involved and to aid in manipulating the events to best advantage. or detrimental. gravitational forces. Overall. The concept of the interfacial region will be presented from a molecular (or atomic) perspective and from the viewpoint of the thermodynamics involved. mechanical motion. if the presence of the interface results in a higher (positive) free energy. the interface will spontaneously be reduced to a minimum—the two phases will tend to separate to the greatest extent possible within the constraints imposed by the container. it is necessary to think in terms of energy.

1a).of new surface formed and the density (i. it can be seen that. it can be seen that the forces acting on the unit are no longer uniform.As will be seen throughout. number) of interfacial units.A.e. along a plane that just touches the unit in question (Fig. and the two new faces are separated by a distanceH. If the bulk phase is cleaved in vacuum. because of their special environment. The actual change in system free energy will also depend on the distance of separation. on average. 2. since unit interactions
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. often possess energies and reactivities significantly different from those of the same species in a bulk or
solution situation.. the unit experiences a uniform force field due to its interaction with neighboring units (Fig.1b). isothermally and reversibly. If one visualizes a unit (an atom or molecule) of a substance in a bulk phase. 2.
The net increase in free energy of the system as a whole resulting from the new situation will be proportional to the area. the unique characters of interfaces and interfacial phenomena arise from the fact that atoms and molecules at interfaces.

When the term ‘‘specific’’ excess surface free energy is used it refers to energy per unit area. giving rise to the concept of ‘‘surface tension. The definition for a curved surface is somewhat more complex. It should be intuitively clear that atoms or molecules at a surface will experience a net positive inward (i. into the bulk phase) attraction normal to the surface.2).e.. but only that part resulting from the units location at the surface. but the difference becomes significant only for a surface of very small radius of curvature. 2. usually in mJ m-2. the resultant of which will be a state of lateral tension along the surface. It should be remembered that the excess free energy is not equal to the total free energy of the system.’’ For a flat surface.will generally fall off by some inverse power law. the surface tension may be defined as a force acting parallel to the surface and perpendicular to a line of unit length anywhere in the surface (Fig.
The specific thermodynamic definition of surface tension for a pure liquid is given by
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.

3a). As a concept. The work of cohesion. which can be interpreted as a two-dimensional analog of pressure (mN m-2 ). when no adsorption of a different material occurs at the surface. Wis the amount of reversible work necessary to overcome the attractive forces between the units at the new surface.’’ is numerically equal to the specific excess surface free energy for pure liquids at equilibrium. The proportionality constant σ.
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. andA is the area of new surface formed. then.Where AH is the Helmholtz free energy of the system. termed the ‘‘surface tension. is defined as the reversible work required to separate two surfaces of unit area of a single material with surface tension σ (Fig. that is.W c. The SI (International System of Units) units of surface tension are mN m -1. surface (and interfacial) tension may be viewed as a twodimensional negative pressure acting along the surface as opposed to the usual positive pressures encountered in our normal experience. 2.

3.e. and (2) chemical units that have a strong attraction for that phase—the lyophilic group (Fig. defined as the reversible work required to separate unit area of interface between two different and 2) to leave surfaces of unit area (Fig..1). the solvent) phase. 2. although the concept is useful for solid surfaces as well. Wa(12).
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.Based on the distinction between solid and liquid surfaces the definition applies strictly to liquid surfaces. normally called the lyophobic group. The work is given by materials (1 two ‘‘bare’’
Surface Activity and Surfactant Structures
Surface-active
materials
(surfactants)
possess
a
characteristic
chemical structure that consists of (1) molecular components that will have little attraction for one surrounding (i. the work of cohesion is simply
Related toW c is the work of adhesion.3b).

for example. but it will often result in the preferential orientation of the adsorbed molecules such that the lyophobic
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. such a distortion (in this case ordering) of the water structure by the hydrophobic group decreases the overall entropy of the system (Fig. 3.
The amphiphilic structure of surfactant molecules not only results in the adsorption of surfactant molecules at interfaces and the consequent alteration of the corresponding interfacial energies.2).In an aqueous surfactant solution.

dispersants. a hydrocarbon.groups are directed away from the bulk solvent phase (Fig. Surfactants may also be generally classified according to some physical characteristic such as it degree of water or oil solubility. or its stability in harsh environments. and at the lowest cost possible. or siloxane chain of sufficient length to produce the desired solubility characteristics when bound to a suitable hydrophilic group. In water.g. Surfactants may be classified in several ways. The chemical reactions that produce most surfactants are rather simple. 3. depending on the intentions and preferences of the interested party (e. The challenge to the producer lies in the implementation of those reactions on a scale of thousands of kilograms. so that surfactants may be classified as emulsifiers. so that it can act as a solubilizing functionality. the author). for example. reproducibly. fluorocarbon.3). or similar. The chemical structures having suitable solubility properties for surfactant activity vary with the nature of the solvent system to be employed and the conditions of use. foaming agents. wetting agents. The hydrophilic (or ‘‘head’’) group will be ionic or highly polar. understandable to anyone surviving the first year of organic chemistry. some specific aspect of the chemical structure of the materials in question may serve as the primary basis for
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. Alternatively. One of the more common schemes relies on classification by the application under consideration. with high yield and high purity (or at least known levels and types of impurity).. the hydrophobic group (the ‘‘tail’’) may be.

) between the hydrophile and the hydrophobe. CH3(CH2)10CH2OH still has very low solubility in water. but the tendency toward solubility has been increased substantially and the material begins to exhibit characteristics of surface activity. If the alcohol functionality is placed internally on the dodecane chain. The four general groups of surfactants are defined as follows: Synthetic surfactants and the natural fatty acid soaps are amphiphilic materials that tend to exhibit some solubility in water as well as some affinity for nonaqueous solvents. insoluble in water. etc. as in 3-dodecanol. however. when the acid is neutralized with alkali it becomes water soluble—a classic soap. If the original dodecanol is oxidized to dodecanoic acid (lauric acid) is CH3(CH2)10COOH the the compound still has limited solubility in water. for all practical purposes.n-dodecanol. nitrogen. the new material. amide. If a terminal hydrogen in dodecane is exchanged for a hydroxyl group (-OH). an example would be the type of linking group (oxygen.
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.classification. The alkali carboxylate will be a reasonably good surfactant. consider the simple. CH3(CH2)10CH a material that is. As an illustration. straightchain hydrocarbon dodecane. the resulting material will be similar to the primary alcohol but will have slightly different solubility characteristics (slightly more soluble in water).

in water the solubility will be determined by the presence of an ionic or highly polar group.
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. Obviously. The most common hydrophilic groups encountered in surfactants today are illustrated in Table 3. to define the complete system under consideration before discussing surfactant types.1.’’ It is important. Commercially there are two main sources for such materials that are both inexpensive enough and available in sufficient quantity to be economically feasible: biological sources such as agriculture and fishing. when neutralized. highly ionizing salts.The solubilizing groups of modern surfactants fall into two general categories: those that ionize in aqueous solution (or highly polar solvents) and those that do not. and the petroleum industry (which is. the definition of what part of a molecule is the solubilizing group depends on the solvent system being employed. For example. The functionality of ionic hydrophiles derives from a strongly acidic or basic character. therefore. leads to the formation of true. while in organic systems the active group (in terms of
solubility) will be the organic ‘‘tail. where R designates some suitable hydrophobic By far the most common hydrophobic group used in surfactants is the hydrocarbon radical having a total of 8–20 carbon atoms. which.

along with some relevant comments about each.
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. ultimately biological).of course. Listed below and illustrated structurally in Figure 3.4 are the most important commercial sources of hydrophobic groups.